A mass spectrometric investigation of the vaporisation behaviour in the (U+Pu+O) system

The vaporisation behaviour of (U,Pu)O2‐x mixed oxides (Pu/M = 0.25, 0.50 and 0.75, with M = U+Pu) was studied by means of mass spectrometry. Hyperstoichiometric samples were heated in a Knudsen cell up to 2300 K. The evolution of the uranium and plutonium bearing gaseous species was studied as a function of time in order to evaluate the congruent vapour composition. Ionisation efficiency measurements were performed and the partial pressures of the species involved in the vaporisation process were determined. The vapour pressure has also been calculated using a thermochemical model for the (U+Pu+O) system. A quasi congruent composition with respect to the O/M ratio has been assumed, in agreement with the experiments. Nevertheless, the overview of all the experimental and calculated results show that a total congruent composition exists for a single composition of the MOX with a Pu/M content slightly lower than 0.50. A good agreement is obtained between the calculated and experimental vapour pressure data, as well as the quasi congruent vaporisation compositions. Keywords: Knudsen cell, mass spectrometry, vapour pressure, thermodynamics, U‐Pu‐O system.JRC.E.3-Materials researc